1. Field of the Invention
The present invention relates to an improvement in a nickel positive electrode used for alkaline storage batteries, such as nickel-cadmium storage batteries and nickel-metal hydride storage batteries. In more particular, it relates to an improvement in sintered nickel substrates used as the electrode base of these batteries.
2. Description of the Related Art
In general, the positive electrode plate used for alkaline storage batteries discharged at a large electric current and charged in a short time, namely storage batteries for so-called high-rate charge and discharge, is a sintered nickel electrode which has a low internal resistance and has excellent cycle life characteristics. The sintered nickel electrode is produced as follows. A sintered nickel porous plaque substrate used as the support for an active material is prepared by coating carbonyl nickel powder on a core material, followed by sintering at high temperature. The substrate is then dipped in an acidic solution of a nickel salt, such as nickel nitrate. Then the substrate is dipped in an aqueous alkali solution to convert the nickel salt impregnating the pores into nickel hydroxide. The above-mentioned series of active material filling operations is repeated several times until a desired amount of the active material is obtained.
The reason for repeating the filling operation is that only one active material filling operation is insufficient to give a desired amount of the active material. Therefore, the desired amount of the active material is filled by repeating the filling operation several times. It is already known, in order to increase the efficiency of filling the active material into the substrate to reduce the number times that the filling operation is preformed and thereby simplify the electrode production steps, to increase the concentration of the nickel salt, e.g. nickel nitrate, used in its aqueous solution thereby increasing the amount of active material filled into the substrate by one filling operation.
The concentration of nickel nitrate in its aqueous solution can be kept high by heating the aqueous solution to a high temperature, whereby the solubility of the salt increases and a specific gravity of the solution of 1.7-1.8 can be reached.
However, an aqueous nickel nitrate solution of such a high concentration has a low pH and hence, owing additionally to its high temperature, is highly corrosive to metals. When a sintered nickel substrate is dipped in such a solution for a long time, the nickel metal constituting the substrate goes into solution by corrosion, and the mechanical strength as a sintered body is decreased.
Therefore, when a battery using a positive electrode plate constituted of a sintered nickel substrate which has received such a treatment is subjected to repeated charge and discharge, since the charge-discharge reaction accompanies the swelling and shrinkage of the active material, the substrate with a lowered mechanical strength is partly broken and/or widened by the swelling of the active material, and hence the nickel electrode readily swells. Consequently, the pore volume of the swollen positive electrode plate increases, the electrolyte held in the separator is taken into the enlarged pores, and the amount of the electrolyte in the separator relatively decreases.
As a result, the separator comes to be exhausted of the electrolyte, which increases the resistance at the time of battery reaction and causes deterioration of the discharge characteristic. The above-mentioned phenomena proceed with the progress of the charge-discharge cycle of the battery, resulting in marked deterioration of the battery capacity.
In recent years, batteries with higher energy density have been eagerly desired and consequently the use of high porosity sintered nickel substrates has become necessary. Accordingly, it has been attempted to attain a high energy density by using a high porosity substrate having a high proportion of pore part obtained by reducing the amount of nickel used, dipping the substrate in an acidic aqueous nickel salt solution of high temperature and high concentration to increase the filled amount of active material, and further conducting the above-mentioned filling operation a number of times.
However, since the amount of nickel constituting such a substrate is small, the sintered substrate has a low mechanical strength. Moreover, since a corrosive, high temperature, acidic aqueous nickel salt solution is used, the thin skeleton of the sintered nickel body is readily deteriorated in its mechanical strength even with a small amount of corrosion, and the electrode prepared by such a method is poor in durability. Accordingly, when the electrode is used to prepare an alkaline storage battery having a high energy density, the battery is liable to undergo marked deterioration of capacity in the charge-discharge cycles of the battery.
To solve the above-mentioned problem, it has been proposed in JP-A-59-96659 to form oxidation-resistant nickel oxide on the surface of sintered nickel of a sintered nickel substrate by heating the substrate at high temperature in the presence of oxygen, in order to prevent the corrosion of the substrate. However, this method also has a problem. When the amount of nickel oxide formed is small a sufficient corrosion preventive effect cannot be obtained and the corrosion of the substrate cannot be suppressed sufficiently. On the contrary, when the amount of nickel oxide formed is increased, the corrosion of the substrate can be suppressed but, since nickel oxide itself is poor in electric conductivity, the increase results, materially, in the existence of a semiconductor or a non-conductor between the active material and the substrate and hence the conductivity between the two is greatly decreased. As a result, the utilization factor of the active material when used in an electrode is lowered.
JP-A-63-216268 discloses a method of preventing the corrosion of a sintered nickel substrate itself by forming a layer of cobalt oxide on the surface of the substrate. In this method, however, a considerable amount of cobalt oxide must be added to suppress the corrosion of the substrate sufficiently, whereas when the amount of cobalt is small the formation of the coating layer is incomplete and the corrosion cannot be prevented effectively.
Thus, in order for the cobalt oxide layer to act effectively for preventing the corrosion of a sintered substrate, it is important that the whole of the sintered nickel porous plaque substrate, including its internal and external skeletons, is coated closely without leaving a gap. When the cobalt oxide layer has a pinhole in some parts thereof, corrosion of the substrate skeleton proceeds from the parts. In a sintered nickel substrate, if even a part thereof is corroded, the potential of the substrate lowers to reach the dissolution potential of nickel, and corrosion proceeds over the whole of the substrate.
For example, one of the methods used for coating the surface of a sintered nickel substrate with a cobalt oxide layer comprises forming on the surface of a sintered nickel substrate cobalt hydroxide in the form of fine particles and then converting the hydroxide into cobalt oxide by heat treatment. Since the reaction is proceeded in the state of particles in this method, pinholes develop inevitably in the cobalt oxide layer. Consequently, the sintered nickel substrate is corroded starting from the pinhole parts of the oxide layer. Protecting the sintered nickel substrate effectively from corrosion cannot be attained unless the cobalt oxide is attached to the substrate in such a large amount as to adversely affect the electric properties of the resulting substrate, particularly the voltage characteristic.
Another method used for forming the cobalt oxide layer comprises dipping a sintered nickel substrate in an acidic aqueous solution of a cobalt salt, such as cobalt nitrate, followed by drying, then dipping the substrate in an aqueous alkaline solution to effect conversion of the cobalt salt into cobalt hydroxide, and then heating the substrate to effect oxidation of the hydroxide. In this way, a cobalt oxide layer can be formed on the surface of nickel constituting the sintered nickel substrate, but the oxide layer formed is sometimes non-uniform and develops pinholes. Moreover, the use of an acidic aqueous cobalt salt solution causes the corrosion of the sintered nickel body in the steps of dipping and subsequent drying.